Windows Vista Taskbar Preview Not Working

I've been using Vista for a few months and one of the features I enjoy is the thumbnail previews of windows on the taskbar. Unfortunately, recently they have stopped appearing and I'm not sure why. When I go to taskbar properties, the option to turn on "window previews" is grayed out as seen in the following screenshot:

When I go to desktop>personalization>window color and appearance, Windows Vista Basic is selected. I then choose Windows Aero and press ok. When I go back to the window color and appearance menu again, though, windows vista basic is again selected. Perhaps this is the root of the problem?

Nevermind, problem solved. Turns out Quake Live (quakelive.com), an in browser game, was taking control of my settings and causing vista to default back to vista basic. Closing the game and closing the browser window restores control to me.